This invention relates to the field, now well established, of use of ansa-metallocenes as catalysts. They are particularly useful as catalysts for the polymerization of ethylene and alpha olefins such as propylene.
Conventional heterogeneous catalysts such as Ziegler-Natta systems have a variety of active sites, only some of which are stereo-specific. Obtaining a polymer with specific properties can involve a considerable amount of trial and error in order to find the best combination of catalyst, co-catalyst and stereo-regulator. In contrast, however, the active polymerization site in a metallocene catalyst is well defined, and can be modified in a straightforward manner via modification of the cyclopentadienyl ligands, enabling the structure of the polymer to be controlled with far greater precision.
A simple metallocene catalyst for polymerizing ethylene is (C.sub.5 H.sub.5).sub.2 ZrCl.sub.2 which consists of a zirconium atom bound to two chlorine atoms and two cyclopentadienyl rings, and which is activated by co-catalysts such as methylaluminoxane (MAO). During the 1980's, ansa or bridged metallocenes, in which the cyclopentadienyl rings are linked by a chemical bridge, were found to be particularly useful for the polymerization of olefins. In particular, ansa-metallocene complexes, when used in combination with a co-catalyst such as methylaluminoxane (MAO), polymerize propylene to highly isotactic polypropylene, highly syndiotactic polypropylene, or atactic polypropylene, depending on the structure of the ansa-metallocene used.
As is well known, isotactic polymers have each pendant group attached to the backbone in the same orientation, whereas in syndiotactic polymers, these groups alternate in their orientations and atactic polymers have a random arrangement of the groups along the backbone. Since the stereochemistry of the polymer has a great effect on its properties, it is desirable to control this feature. Chiral, C.sub.2 -symmetric ansa-metallocenes produce isotactic polypropylene.
While the greatest area of potential use for ansa-metallocene catalysts currently is for polymerization of olefins, such as ethylene and propylene, they also have significant uses as catalysts or catalyst precursors for other reactions where stereo-selectivity is important.
The utility of ansa-metallocene complexes as catalysts for olefin polymerization and other reactions has created a high demand for a practical synthesis of ansa-metallocene compounds.
In spite of this demand, current procedures for the synthesis of Group 4 (Ti,Zr,Hf) ansa-metallocenes based on the use of ansa-bis-cyclopentadienyl dianion reagents are hampered by low yields and tedious isomer separation and purification steps. Some of these problems have been discussed in Ellis, W. W.; Hollis, T. K.; Odenkirk, W., Whelan, J.; Ostrander, R.; Rheingold, A. L.; Bosnich, B. Organometallics 1993, 12, 4391. In particular, the synthesis of chiral C.sub.2 symmetric ansa-metallocenes typically produces mixtures of desired rac (racemic) and undesired meso isomers. A typical synthesis of an ansa-metallocene complex is shown in equation 1 below: ##STR1##
This equation is typical of the process as shown in the art. See for example Spaleck, W.; Kuber, F., Winter, A.; Rohrman, J.; Bachmann, B.; Antberg, M.; Dolle, V.; Paulus, E. F. Organometallics 1994, 13, 954. Stehling, U.; Diebold, J.; Kirsten, R.; Roll, W.; Brintzinger, H. H.; Jungling, S.; Mulhaupt, R.; Langhauser, F. Organometallics 1994, 13, 964. Halterman, R. L. Chem. Rev. 1992, 92, 965. See also, for example, U.S. Pat. No. 5,145,819, U.S. Pat. No. 5,268,495, and EPA 0-530-908-A1.
By way of further example, an important chiral Group 4 ansa-metallocene is rac-(EBI)ZrCl.sub.2 (EBI=ethylene-1,2-bis(1-indenyl) which is currently prepared from ZrCl.sub.4 and the dianion of the EBI ligand (Halterman, R. L. Chem. Rev. 1992, 92, 965). Brintzinger (Wild, F. R. W. P.; Wasiucionek, M.; Huttner, G., Brintzinger, H. H. J. Organomet. Chem. 1985, 288, 63) and Collins (Collins, S.; Kuntz, B. A.; Hong, Y. J. Org. Chem. 1989, 54, 4154; Collins, S.; Kuntz, B. A.; Taylor, N. J.; Ward, D. G. J. Organomet. Chem. 1988, 342, 21) used (EBI)Li.sub.2 and reported low, variable yields (20-50%) of rac-(EBI)ZrCl.sub.2. Buchwald employed (EBI)K.sub.2 and obtained (EBI)ZrCl.sub.2 in a rac/meso ratio of 2/1 in 70% yield. (Grossman, R. B.; Doyle, R. A.; Buchwald, S. L. Organometallics 1991, 10, 1501). In general, these salt elimination procedures produce the desired rac ansa-metallocenes in 10%-30% yield after tedious separation and purification steps, and the separation of the rac from the meso products is not always possible.
Ansa-metallocenes can also be prepared by amine elimination reactions of ansa-bis-cyclopentadienes and metal amide complexes as disclosed in U.S. Pat. No. 5,597,935. An example is the reaction of Zr(NMe.sub.2).sub.4 with 1,2-bis(3-indenyl)ethane ((EBI)H.sub.2), shown below (Eq. 2). This reaction provides an efficient, high-yield synthesis of pure rac-(EBI)Zr(NMe.sub.2).sub.2, which can easily be converted to rac-(EBI)ZrCl.sub.2 and related derivatives. ##STR2##
However, this amine elimination reaction produces the noxious gaseous amine NMe.sub.2 H as a byproduct which must be removed and disposed of. Additionally, the amine elimination approach is less useful for the synthesis of Ti or Hf ansa-metallocenes (order of reactivity: Zr&gt;Hf&gt;Ti) and does not work well with crowded metal amides, or crowded or weakly acidic cyclopentadienes. For example (EBI)H.sub.2 does not react with Ti(NMe.sub.2).sub.4 in toluene at 100.degree. C. (Diamond, G. M.; Jordan, R. F.; Petersen, J. L. Organometallics 1996, 15, 4030-4037). Also, the reaction of Hf(NMe.sub.2).sub.4 with (1-indenyl).sub.2 SiMe.sub.2 proceeds only under vigorous conditions (neat, 32 h, 120.degree. C.) to give rac-{(1-indenyl).sub.2 SiMe.sub.2 }Hf(NMe.sub.2).sub.2 in 20% isolated yield, and the rearrangement product Me.sub.2 Si(.eta..sup.5 -1-indenyl)(.eta..sup.3 -2-indenyl)HF(NMe.sub.2).sub.2 is observed as a side product (Christopher, J. N.; Jordan, R. F.; Petersen, J. L.; Young, V. G. Jr. Organometallics 1997, 16, 3044-3050).
Kunicki et al. reported that the simple unlinked metallocene (C.sub.5 H.sub.5).sub.2 TiCl.sub.2 can be prepared by the reaction of {(C.sub.5 H.sub.5).sub.2 Al(O.sup.i Pr)}.sub.2 with TiCl.sub.4 (Kunicki, A.; Sadowdki, R.; Zachara, J. J. Organometal. Chem. 1996, 508, 249). Ansa-metallocene complexes are not discussed in the Kunicki reference.
There is, therefore, a need for a process which would produce ansa-metallocene complexes in high yield without the formation of noxious gaseous byproducts. Additionally, there is a need for a process which would produce rac ansa-metallocenes in high yield without contamination by the meso isomer, since the rac isomer is most useful in stereoselective catalysis. The present invention has as its primary objectives the fulfillment of these needs.